Adapters of this type are used, for example, to draw single-phase current from a bus bar having a neutral conductor in addition to three phase conductors. A selector switch may be used for completing the circuit to any one of these phase conductors, e.g. as described in German published specification 2,411,976. This prior-art device comprises two coaxially intercalated control shafts, one of them serving to clamp the adapter to the bus bar while the other has blades engageable on the one hand with the neutral conductor and on the other hand with the selected phase conductor. The resiliency of the blades facilitates their rotation, with the corresponding shaft, into an engagement position; the two shafts interlock in such a way that closure of the circuit becomes possible only when the adapter is firmly clamped whereas its withdrawal from the bus bar is prevented until the contact blades have been returned to a disengagement position.
A drawback of devices so constructed is that the presence of the aforementioned selector switch, in series with the contact blades designed to engage the phase conductors, unavoidably introduces an additional circuit resistance giving rise to a significant voltage drop which, particularly with loads drawing large currents, not only wastes energy but results in considerable evolution of heat.